RESPA-CFX, the second component of a complete cab air quality system, provides powered, high-efficiency recirculation filtration with your choice of included MERV16 (F9) or HEPA (H13, ISO35H) filter. Some assembly required. RESPA-CFX is the recommended recirculation choice when you are unable to plumb directly into the HVAC plenum; use non-powered RESPA-FFX2 if you can plumb into plenum.